Optical communications systems using optical fibers to connect a light source and detector or regenerators have reached a high state of technological development with data rates in excess of 10 Gbits/sec and transmission spans of more than a 1000 kilometers possible. A transmission span is the distance between light source and detector or regenerators. The entire transmission distance frequently includes more than one span. One system parameter that has been of interest in attempts to increase system capacity is chromatic dispersion. The presence of chromatic dispersion means that light at different wavelengths or frequencies travels at different velocities in the optical fiber.
Chromatic dispersion, even for very brief pulses, can limit either the operating frequency, that is, the data transmission rate, or the transmission span of the system. Although fibers can be fabricated with low dispersion, some dispersion typically remains because the fabrication does not totally eliminate the chromatic dispersion. Accordingly, techniques have been developed to compensate for fiber chromatic dispersion. One dispersion compensation technique passes the signal through a dispersion compensating fiber (DCF) as well as the system fiber. The DCF has a dispersion opposite to that of the system fiber. The DCF is desirably selected, with respect to length and dispersion, so that the total dispersion for the signal passing through the DCF compensates for the dispersion of the signal as it passes through the system fiber.
This approach is used in U.S. Pat. No. 5,218,662 issued Jun. 8, 1993 to Dugan. The patent describes a system having a plurality of transmission
spans with a dispersion compensation unit present for each span. Each dispersion compensation unit reduces the dispersion for a single span. This approach was adopted by Dugan because he believed that compensation in the field required large numbers of fibers of different lengths, and was therefore impractical. The system described by Dugan is static; that is, the amount of compensation can not be readily changed and there is no feedback, while the system is operating, between the receiver and the dispersion compensation units about the quality of the received signal.
While the system described may operate well for many applications, it has drawbacks. In particular, in many systems, signals from one transmitter may, at different times, be transmitted over different system fibers requiring different amounts of chromatic dispersion. That is, the system configuration is dynamic and changes in time as system usage changes. The system described can not vary the amount of chromatic dispersion used to compensate for the dispersion in the system fiber. Additionally, the total transmission path can include a plurality of spans, and the system described can not easily compensate for the chromatic dispersion over a plurality of spans but rather considers and compensates for each span individually.